Black Mold: What It Is, What It Isn't, and When to Worry

Few phrases cause as much worry as “black mold.” Some of that worry is justified, and some of it isn't. Here's what the term means, who should be most careful, and why the right response depends on where mold is growing — not on its color.

General information only, not medical advice.

What People Mean by “Black Mold”

Usually, Stachybotrys chartarum— a dark greenish-black mold that grows on materials rich in cellulose, like drywall paper, ceiling tiles, and wood, after they've stayed wet for an extended period. It's associated with long-running leaks and water damage that wasn't dried.

But many other molds are dark too. The black spots on shower caulk or window tracks are usually other species that thrive on surface moisture and soap residue. You can't identify mold by looking at it — only a lab can.

Who Is Most Sensitive

Public health agencies generally note that mold exposure can cause stuffy nose, wheezing, coughing, and eye or skin irritation in sensitive people, and can worsen asthma. People more likely to be affected include:

• People with allergies or asthma.
• Infants and young children.
• Older adults.
• People with weakened immune systems or chronic lung disease.

Why Color Shouldn't Decide Your Response

Any substantial indoor mold growth deserves attention, whatever its species. The practical questions are the same:

• How large is the affected area?
• Is it on porous materials like drywall, or hard surfaces like tile?
• Is it inside walls, ceilings, or the AC system?
• What water source is feeding it?

Small areas on hard surfaces can usually be cleaned by a homeowner. Larger areas, growth on drywall or ceilings, and anything in the HVAC system call for a professional with containment. See black mold removal.

When Identification Is Worth It

Lab identification is useful when the result matters to someone else — a physician, a landlord, a buyer, or an insurance adjuster. A licensed assessor can take surface or air samples and interpret them properly. See mold testing.

What Not to Do

• Don't scrape, sand, or dry-brush large areas of growth.
• Don't run fans that blow across moldy surfaces into the rest of the house.
• Don't paint over it — the growth and moisture remain.
• Don't rely on bleach for drywall or wood.

If you're seeing dark growth after a leak or storm, start with a mold inspection to find the source and the full extent.